Is this even a bug?

with glibc 2.28 mmap-ing 1 byte (in a kernel with page_size 4096 bytes) like:
`addr = mmap (NULL, size, PROT_READ, MAP_ANONYMOUS | MAP_PRIVATE, -1, 0);`
... allows accesses until addr[16384-1] and only segfaults at accessing addr[16384] (page 4 instead of 2)

but with glibc 2.29, segfault happens only at addr[188416]  (page 46 instead of 2)

for details see comment 14 & 15 in: https://bugzilla.kernel.org/show_bug.cgi?id=203537#c14


Note that I tried this with MAP_FILE (and an actual file) not just with MAP_ANONYMOUS but I found that the effects/results are the same.

I would expect segfault(or SIGBUSS? according to the man page) would happen when hitting page 2 (ie. addr[0x1000]) but this would seem to require something like what valdis mentioned: " * valdis wonders how hard it would be to add an "allocate non-accessible guard page at both ends" flag to mmap()."

So, is the manual in need of updating? since SIGBUS isn't generated as stated in these circumstances:

man 2 mmap:
    SIGBUS Attempted access to a portion of the buffer that does not correspond to the file (for example, beyond the end of the file, including the case where another process has truncated the file).

man mmap (ie. map 3p mmap):
    The system shall always zero-fill any partial page at the end of an object. Further, the system shall never write out any modified portions of the last page of an object which are beyond its end. References within the address range starting at pa and continuing for len bytes to whole pages following the end of an object shall result in delivery of a SIGBUS signal.
    An implementation may generate SIGBUS signals when a reference would cause an error in the mapped object, such as out-of-space condition.

(pa=addr and len=size in my case)

At the very least, SIGBUS isn't generated beyond the end of the file until the end of the page.

Note: SIGBUS is generated when file is a 0 byte file (from what I've tested) and this is to be expected and working normally (now that I know enough to understand it).

For a 200MiB mmap, both glibc versions allow accesses until page 442 beyond mmap-ed region is reached (page 443 for v2.29) and only then segfault.